The invention relates to a circuit arrangement and to a method for pulsated charging of batteries which scan be used practically to all kinds of batteries.
For charging batteries, especially in case of using cheaper charger circuits, a direct current voltage is used which has been rectified from the alternating current line voltage, and the direct current voltage is not smoothed, thus the charging current follows the pulsation of the rectified voltage. It is known that the charging with a pulsating current is not disadvantageous, when during the charging process certain limit values, characteristic to the actually used battery are not exceeded. The properties of the charging with a fluctuating and pulsating current have not been noticeably different from those at charging with smoothed direct current. Such a property is e.g. the cycle life of the battery, that corresponds to the number of charging and discharging cycles, within which the capacity of the battery does not decrease below a predetermined fraction of the initial capacity, for instance to 60%. Another important feature is the capacity expressed in ampere-hour units, which is initially high and decreases with the number of cycles. The cumulated capacity is not else than the extent of the whole energy delivered by the battery during the full cycle life. In addition to the above described battery characteristics, the battery cam also be characterized by the time required for attaining the fully charged state, the increase of temperature during the charging and discharging process, the peak value of the current that can be taken out of the battery, furthermore the appearance of the memory effect, finally the probability of accidental short circuits.
It is a generally accepted experience that the above listed individual properties cannot be improved without causing a worsening of one or more of the other parameters. When the charging time is decreased, the cycle life generally increases, the reliability decreases, and the capacity of the battery cannot be utilized either.
In conventional battery charger circuits the circuit portions that generate the charging current are very similar, they comprise a full wave rectifier connected to the secondary winding of a line transformer, and the direct current ports of the rectifier are connected to the battery terminals. The electronic circuits used in such chargers have the main task of monitoring the parameters of the batter during the charring process, on the basis of which the end moment of the charging is determined. In case of charging with a pulsating direct current, problems might rise if the internal resistance of the supply source is very low, because in such cases the difference between the almost constant battery voltage and the peak of the charging voltage might result in very high peaks in the charging current that the battery might not endure, or it is very difficult to adjust the optimum value of the charging current. The problem gets more difficult by the fact that during the charging process the battery voltage increases and the charging parameters cannot by adjusted accordingly. This problem is rarely apparent because the power supplies used in everyday practice have internal resistance values much higher than required, and the high internal resistance prevents the formation of high charging current peaks. This property is favorable from the aspect that it suppresses the above problem, but at the same time it is disadvantageous because the battery will not charged by current and voltage values that were otherwise required for obtaining an ideal charging. This will manifest itself by the longer charging time, by the appearance of the memory effect and by the decrease of life time, thus by the less favorable values of the battery parameters compared to the values determined theoretically by the battery design.
In U.S. Pat. No. 4,878,007 a pulse charging for nickel-cadmium batteries has been suggested, wherein respective short discharging sections were inserted between subsequent charging pulses. This charging method resulted in an increased activity in the internal chemical processes of the battery, and as a result the memory effect decreased, moreover the batteries with previously decreased capacity could be regenerated. In the practice the suggested way of charging has not proven much better than available charging methods, since the use of the steep charging pulses decreased the life time of the batteries, furthermore it was difficult to realize the required charging-discharging cycles.
U.S. Pat. No. 5,463,304 describes a life extending circuit for storage batteries that comprises a capacitor connected in series with the primary winding of the line transformer of an AC source tuned to form a resonant circuit with the inductance of the primary winding. This circuit provided some extension of the life cycle of the batteries, but it could not substantially influence the process of charging, since the DC level at the secondary winding of the circuit has remained unchanged, and the properties of the charging current (especially the peak value) were limited by the limiting properties of the transformer and the associated circuitry.
If one wishes to obtain a substantial improvement in the charging of batteries compared to conventional methods, if that objective can be attained at all, the processes that takes place in the battery should be studied more carefully, because on the basis of such studies one might draw conclusions that show the way towards obtaining better parameters.
In the book of Dr. Hevesi, Imre. xe2x80x9cElektromossxc3xa1gtanxe2x80x9d (in English: Theory of Electricity) published by Nemzeti Tankxc3x6nyvkiadxc3x3, Budapest, 1998, on pages 428-429 the movement of ions in electrolytes is described. It is stated that ions have finite velocity of displacement which gets stabilized following a voltage has been applied, and the velocity is directly proportional with the intensity of the established field. The velocity depends also on the charge of the ions and a friction coefficient a that act against the movement of the ions.
The primary objective of the present invention is to provide a charging method and a circuit arrangement implementing the method that can provide more favorable charging conditions, and as a result of which the battery parameters will significantly improve compared to conventional charging methods.
For attaining this objective and based on the above cited literature, it has been supposed that the chemical reaction at the close vicinity of the electrodes will take best place in a time period when the ions from the other electrode have not yet arrived and their presence cannot disturb the processes taking place in this electrode region. This condition prevails at the initial period of the establishment of the electrical field, and when the formation of a balanced state of the ions at the electrode region is prohibited for short time periods.
For this purpose and based on this supposition a circuit arrangement as claimed in claim 1 to 11 has been provided.
According to a further aspect of the invention a method has been provided for pulse charging batteries by using an alternating current source, which method is defined in claims 12 to 14.